1. Field of the Invention
The present invention relates to a die-casting method and device particularly useful for the die-casting of an aluminum alloy.
2. Description of the Related Art
In a conventional die-casting, after a movable mold is released from a stationary mold and an article is removed from the molds, a molding release agent, i.e., a lubricant, is blown on surfaces of the molds as lubrication, so that adhesion of molten aluminum to the molds is reduced and thus the die-casting process can be carried out continuously. The molding release agent is blown onto cavities formed on the molds for die-casting the article to a desired shaped form, especially on portions of such cavities where molten metal is introduced through passages formed in the molds, and on portions having a complex configuration. With such a construction, if a plurality of articles are to be molded at the same time, the molding release agent must be blown onto the cavities for a long time, thus lengthening the cycle time and lowering the productivity.
In an attempt to solve this problem, Japanese Unexamined Patent Publication Nos. 60-49851 and 61-20654 disclose methods in which the lubrication of the molds cavities is carried out by providing an injection sleeve having a plunger, for supplying the lubricant into the cavities.
This conventional die-casting method, however, has the following problems. Namely, the lubricant applied to the movable and stationary molds becomes mixed with the molten metal while the molten metal is being injected into the mold cavities, and since the lubricant becomes liquid or gaseous during this mixing, the lubricant mixed in the molten metal may expand when the solidified article is heated during a later process (for example, in a heat treatment such as an aging treatment after a solution heat treatment), thus causing a deformation of the article.
To solve this problem, the present inventors disclosed, in Japanese Unexamined Patent Publication No. 62-156063, a method and a device in which the lubricant is blown on a portion in the mold having a highest temperature, to be decomposed before being introduced into the mold cavities so that the surfaces of the molds are lubricated. In this method, the article is not deformed even when subjected to a heat treatment after solidification. Nevertheless, although this method and device are satisfactory if the number of articles to be die-cast at one time is relatively small and sufficient lubrication is obtained, if the number of articles to be die-cast at one time is relatively large, the amount of lubricant supplied into each mold cavity is different in each cavity. Therefore, in a cavity where the amount of the supplied lubricant is small, molten aluminum adheres to the surface of a complex shape cavity portion after each molding, and if the amount of lubricant supplied to the cavity is too large, the lubricant may not be fully decomposed.